Aromatic polyamides have been known for a substantial period of time and are usually prepared by the reaction of an aromatic diamine with a dibasic acid or derivative. Aromatic polyamides have been used in various applications including electrical/electronic applications in various forms but generally not as transparent, high Tg, amorphous films or coatings. Most commercial aromatic polyamides presently are crystalline materials e.g. aromatic polyamides based on isophthalic acid and m-phenylenediamine; the liquid crystalline aromatic polyamides are based on terephthalic acid and p-phenylenediamine. These crystalline aromatic polyamides have excellent properties but films can only be cast from very strong (and often toxic) solvents such as anhydrous (or fuming) sulfuric acid, anhydrous hydroluoric acid, chlorosulfonic acid, fluorosulfonic acid, or hexamethyl phosphoramide and the resultant films are not transparent due to the resultant crystallinity.
Poly(amide-imides) have also been utilized as films for electrical/electronic applications and are formed by the reaction of trimellitic anhydride with methylene dianiline). The poly(amide-imide) films are amorphous, transparent, and have high Tg's--but are based on high cost monomers. Another class of materials termed transparent polyamides, which are amorphous, exist and are utilized for injection molding applications. These polymers contain aliphatic carbon units in the main chain and are thus limited in thermal stability. Additionally, they exhibit glass transition temperatures of 150.degree. C. thus cannot be considered for high performance electrical/electronic film applications.
There is substantial patent literature regarding polyamide resins and their preparation. for example, the early polyamides such as those disclosed in U.S. Pat. No. 2,130,948 had low melting points and poor thermal stability in air at elevated temperatures. In the early 1960's, several patents assigned to the duPont Company, e.g., U.S. Pat. Nos. 3,006,899; 3,063,966; and 3,094,511 disclose processes for preparing aromatic polyamides and discuss two conventional procedures for producing the aromatic polyamides. Melt polymerization involving high temperature polymerization, up to 300.degree. C., was suggested as one type but was objectional since water-white polymers could not be prepared at such elevated temperatures. Low temperature polymerization was suggested for reducing degradation associated with melt polymerization but reaction between the acid halide and solvent interfered with the production of high molecular weight polymers.
U.S. Pat. No. 3,006,899 discloses the preparation of film and fiber from high molecular weight wholly aromatic polyamides by carrying out the interfacial polymerization of an aromatic diamine with an aromatic diacid chloride in the presence of a cyclic, non-aromatic oxygenated organic solvent, e.g., tetrahydrofuran, tetramethylene sulfone, and cyclohexanone. Inorganic alkaline materials such as sodium carbonate were preferred as acid acceptors although organic acid acceptors such as triethylamine were alleged as being suited for producing such high molecular weight polyamides. Example III shows a reaction product of 2,4-diaminotoluene with terephthaloyl chloride and the polymerization of such materials to yield a polymer having an inherent viscosity of 1.37 (measured in concentrated sulfuric acid) at 30.degree. C. at a concentration of 0.5 grams per 100 cc of solution.
U.S. Pat. No. 3,063,966 discloses a process for making wholly aromatic polyamides using low temperature reactions, e.g., below 50.degree. C. A halogenated non-aromatic hydrocarbon was used as a solvent and organic tertiary amines were used as acid acceptors in preparing the polymer. Amide type organic compounds, e.g., dimethylacetamide, N-acetyl pyrrolidine and N-dimethylpropionamide were suggested as being suitable solvents. Aromatic polyamides having an inherent viscosity of at least 0.6 in sulfuric acid and a melting point of at least 300.degree. C. were prepared.
U.S. Pat. No. 3,094,511 discloses a process for producing wholly-aromatic polyamides having an inherent viscosity of at least 0.6 and water-white clarity with a melting point above about 300.degree. C. by effecting reaction between an aromatic diamine and an aromatic diacid halide. As acknowledged at columns 5 and 6, amorphous and crystalline fibers or films are produced, but crystalline polymers were preferred for their retention of outstanding physical properties.
Two patents assigned to the Teijin Company disclose processes for preparing aromatic polyamides by reacting substantially equal molar amounts of diamine with a dihalide in an inert, nonbasic organic medium to form a precondensate and contacting the precondensate with an aqueous solution of water-soluble acid acceptor. These patents are as follows:
U.S. Pat. No. 3,640,970 initially discloses polyamide preparation by the interfacial polycondensation method of U.S. Pat. No. 3,006,899 and the low temperature polycondensation method described in U.S. Pat. No. 3,063,966 and problems associated with these procedures. The '970 patent then disclosed a precondensate method for polyamide preparation and suggested it was preferred to interfacial polycondensation method because it permitted better control of molecular weight of the product. With interfacial polymerization the degree of polymerization was susceptible to slight changes in the stoichiometric conditions of reactants. Low temperature solution polymerization techniques were difficult because of the selection of solvent and the separation of product from the solution. The latter problem was compounded because of by-product formation. In the '970 patent, a precondensate technique was disclosed wherein an aromatic diamine was reacted with an aromatic dicarboxylic acid dihalide in a polar, nonbasic, inert organic liquid medium under conditions such that a condensation product having a low degree of polymerization was formed. Ethers, ketones, sulfones, and halogenated hydrocarbons were suggested as being suitable solvents. After the precondensate was formed, the organic liquid medium containing precondensate was contacted with an aqueous solution of a water-soluble acid acceptor to complete the intended polyamide-forming reaction. Water-soluble acid acceptors suited for neutralizing by-product hydrohalide acid included inorganic and organic alkali-metal hydroxides, carbonates and bicarbonates as well as organic amines, e.g., triethylamine and triethylenediamine. Inherent viscosities of 0.6 to 3 in sulfuric acid were deemed possible.
U.S. Pat. No. 4,009,154 discloses a precondensate process for preparing aromatic polyamides using sodium carbonate hydrate as the acid acceptor. The patentees acknowledge that the two-stage process as described in the '970 patent, or, as sometimes referred to as the oligomer polymeriation process, permitted one to control the degree of polymerization more readily on a large scale. The patentees then noted that through the use of an acid acceptor in the form of sodium carbonate hydrate in an aqueous slurry, as opposed to an aqueous solution of an alkali compound, it was easier to obtain a high degree of polymerization. Other acid acceptors and their chracteristics were discussed. For example, when alkali metal hydroxides were used, the polymerization reaction system became strongly alkaline and hydrolysis of the carboxylic acid halide often took place. It was because of hydrolysis of the halide that it was difficult to obtain a high degree of polymerization. Alkaline earth metal hydroxides formed salts which were difficult to remove from the polymer. Organic amines such as triethylamine were often unsuited because of the difficulty in obtaining a high degree of polymerization. Example 25 of the '154 patent shows a process for preparing a polyamide of 2,4-toluenediamine and isophthalic acid chloride in cyclohexane and the polymerization subsequently finished by adding sodium carbonate and sodium chloride in water to the reaction product. The inherent viscosity of the polymer was 1.86.
German Pat. No. 26,06,878 discloses a process for the preparation of powdered aromatic polyamides. The process uses a solvent for the monomers but a nonsolvent for the polymer. In this process the aromatic diamine is reacted with an aromatic dicarboxylic acid in the absence of an acid acceptor and then reacted under anhydrous conditions in the presence of a tertiary amine in sufficient amount to neutralize the hydrogen halide formed. Examples of solvents for the polymerization include methylene chloride, chloroform, acetonitrile, dioxane and others. Example 2 shows the reaction of 2,4-diaminotoluene with isophthaloyl chloride in the presence of methylene chloride and chloroform. After all of the diamine is charged, the thus formed precondensate was contacted with triethylamine hydrochloride and triethylamine under reflux conditions. A polymer powder having a relative viscosity of 2.03 in N-methyl pyrrolidone was obtained.